The vehicle information and communication system (VICS) is known as a conventional service for broadcasting traffic information. The present service is implemented to provide various kinds of traffic information and vehicle information to a user. For example, the VICS Center is caused to transmit traffic information, such as traffic congestion information about the road, to a vehicle. In addition, a vehicular device is caused to search map data for identifying a road. Furthermore, a display device is caused to change a display mode of a road according to the received traffic information. The present service enables a user to obtain traffic information such as traffic congestion information in real time.
A vehicular device stores map data including road data in a format defined with links and nodes. A link represents a road having nodes being termination points. The VICS Center transmits the VICS Link being information for identifying roads. The VICS link is assigned with various traffic information and change instruction information on a display mode. A vehicular device has a position reference table for comparing the VICS Link with links in the map data. The vehicular device searches a link corresponding to the VICS Link with reference to the table. That is, the position reference table is requisite for the VICS system (see, for example, JP-A-2006-275777 and JP-A-2009-270953).
As an alternative service to the VICS system, it is conceived to utilize data of traffic information transmitted in the form of transport protocol expert group (TPEG) to a terminal device such as a vehicular device. It is noted that in the case of TPEG data being transmitted, position information is represented in the form of, for example, dynamic location referencing data (DLR data). The position information includes core points each having position coordinates and attributes for identifying a road. In general, the core point is distributed in the form of multiple arrays arranged along the road. In the system where the core points are used to represent position information, a position reference table, which may vary in dependence upon difference in manufacturer of the map data, the format and the version of the map data, and the like, is unnecessary. That is, the system using the core points enables identification of a road (link) on the map data, regardless of the map data in the vehicular device.
To the contrary, the system using the core points needs various processings for identifying a road according to the core points. For example, as described above, various kinds of map data exist. Therefore, core points do not necessarily exist on a road of map data. Therefore, it is necessary to implement a processing to identify a link pertinent to a road represented by core points on a map.
More specifically, it is necessary to extract a candidate link on the map data for estimating a road represented by core points. Therefore, it is conceived first to extract a link near the core points.
It is noted that core points forms a discrete array. Therefore, it is impossible to represent a continuous road with only links merely located near the core points. That is, in order to estimate a road represented by core points, it is an essential subject to employ a method for extracting links near a core point and a method for further extracting a road (link) connecting the extracted links.